The present invention relates to a semiconductor memory device having stack type memory cells.
In a conventional semiconductor memory device having stack type memory cells, a dielectric film is formed between storage nodes and plate electrodes to form memory cell capacitors.
FIG. 15 shows a part of the structures of conventional stack type memory cells. Referring to FIG. 15, a plate electrode 143 is formed above storage nodes 141, with a dielectric film 142 interposed between the plate electrode 143 and the storage nodes 141. After these elements are formed by a manufacturing process such as spin coating, sputtering or CVD (chemical vapor deposition), the plate electrode 143 is covered by a resist 144, and then is processed by a dry-etching technique such as RIE (reactive ion etching) or CDE (chemical dry etching) using charged particles 145, in order for the plate electrode 143 and the dielectric film 142 to have predetermined shapes.
The dry etching technique uses the charged particles 145 accelerated due to an electric field, and makes the charged particles 145 directly collide with the plate electrode 143. In the etching, the plate electrode 143, as shown in FIG. 16, is etched from its surface to a predetermined depth. Since the plate electrode 143 is electrically floated, it is charged up during the etching process. In other words, charges are accumulated in the plate electrode 143.
The accumulated charges move within the plate electrode 143 since the plate electrode 143 is formed of a conductive material. Consequently, electrical stress or a high voltage acts on the entire dielectric film 142, regardless of the distance from the processed (dry-etched) portion of the plate electrode 143. Therefore, there is a case where discharge as represented by reference numeral 146 occurs in the dielectric film 142, and damages it.
Furthermore, after processing the plate electrode 143 by the dry etching technique, an interlayer insulating film 147 formed of, e.g., SiO.sub.2 is provided on the plate electrode 143 as shown in FIG. 17. Provision of the interlayer insulating film 147 is intended for formation of a contact. To be more specific, a contact hole 148 is formed in the interlayer insulating film 147, in order to connect a voltage generating circuit (not shown) and the plate electrode 143. Due to this structure, a given potential can be applied to the plate electrode 143 when the semiconductor memory device is operated.
The contact hole 148 is formed by the dry etching technique in the same manner as the plate electrode 143 shown in FIG. 16. In this case, the contact resistance is high if a part of the interlayer insulating film 147 is remained in the bottom of the contact hole as shown in FIG. 17. Therefore, etching must be sufficiently performed. To be more specific, dry etching is not stopped even after the contact hole 148 reaches the plate electrode 143. In other words, etching is performed until the plate electrode 143 is shallowly cut, i.e., over-etching is performed.
At this time also, the plate electrode 143 is charged up, as a result of which the dielectric film 142 is damaged.
Furthermore, after the contact hole 148 is formed above the plate electrode 143, it is filled with a conductive material, thereby forming a contact (not shown). Then, an upper wiring layer is also processed by dry etching after it is formed in such a manner a way to be connected to the contact. In this case also, the plate electrode 143 is charged up due to the charge passing through conductive members such as the wiring layer and the contact, and thus the dielectric film 142 of capacitors is damaged.
The memory device can be made smaller and operate at a higher speed, if the dielectric film 142 is formed of a material having a high dielectric constant and formed to have a small thickness. However, this cannot be achieved in the above conventional memory device due to the above-mentioned charge up problem.
Therefore, the present invention has been made in consideration of the above circumstances, and its object is to provide a semiconductor memory device which is made smaller and operates at a higher speed due to the following feature:
The plate electrode of cell capacitors is prevented from being charged during the dry etching process, thus preventing damage of the memory device.
By virtue of this feature, the yield and the reliability are improved, and a dielectric film is made thinner, by using a plurality of dielectric materials having high dielectric constant.